Blender resizing my image in half
Check Render Resolution Percentage

Factory settings have a percentage of the render size set to 50%, that means the to the values in the upper fields , a multiplyer of 0.50 units will be applied.



This feature it's very handy when it comes to build a quick preview keeping the final output proportions.

I you would like to render in full resolution just enter 100% as percentage.

Note: beware that the field accepts values also above 100. Count wisely the number of zeros.